As, for example, shown in FIG. 1, a signal received by an antenna 1 is supplied through a tuner 2 and a video intermediate frequency amplifier 3 to a video signal detecting circuit 4 by which a video signal is detected. This video signal is supplied through a delay circuit 5 the delay time of which corresponds to the period of duration necessary for cancelling out a pre-ghost signal to a composer 6. Also, a ghost imitating signal from a transverse filter which will be described later is supplied to the composer 6 and thus the composer 6 supplies therefrom a video signal whit a ghost signal eliminated to an output terminal 7.
The video signal derived from the video signal detecting circuit 4 is supplied to a delay circuit 8 which constructs the transversal filter. The delay circuit 8 is formed such that delay elements each of which has a sampling period (for example, 10 nano seconds) taken as a unit of delay are connected in plural stages (n in number) so as to achieve a delay time equal to the pre-ghost signal eliminating period and n taps are led out from the respective stages. The signals from the respective taps are supplied to weighting circuits 9.sub.1, 9.sub.2, . . . 9.sub.n, each of which is formed of a multiplier.
The signal from the last stage of the delay circuit 8 is supplied to a terminal 10f of a mode switch 10, while the output signal from the composer 6 is supplied to a terminal 10b of the switch 10. The signal from the switch 10 is supplied to a delay circuit 11. The delay circuit 11 is formed such that delay elements, each of which has a sampling period taken as a unit of delay, are connected in plural stages (m in number) so as to achieve a delay time equal to the delay-ghost eliminating period and m taps are led out from the respective stages. The signals from the respective taps are supplied to weighting circuits 12.sub.1, 12.sub.2 . . . 12.sub.m, each of which is formed of a multiplier.
The video signal from the composer 6 is supplied to a subtracting circuit 13. The video signal from the delay circuit 5 is supplied to a synchronizing separating circuit 14 and the separated vertical synchronizing signal therefrom is supplied to a standard waveform generating circuit 15 and a low-pass filter 16 wherein a standard waveform approximate to a step waveform of a rising edge VE of the vertical synchronizing signal is formed. The standard waveform is supplied to the subtracting circuit 13.
The signal from the subtracting circuit 13 is supplied to a differentiating circuit 17 which detects the ghost signal.
As a signal for detecting and measuring the ghost signal, used is such one that is contained in a standard television signal and not affected by other signal during a period of a duration as long as possible, for example, the vertical synchronizing signal. That is, as shown in FIG. 2, the portion between the rising edge VE of the vertical synchronizing signal and the period of .+-.1/2H (H represents the horizontal period) before and after it is not affected by other signal. Therefore, the afore-said standard waveform is subtracted from the signal of such period and the subtracted signal is differentiated to thereby detect a weighting function.
If a video signal contains a ghost signal having, for example, a delay time .tau. and a phase difference .phi. (=.omega..sub.c .tau. where .omega..sub.c is a video carrier angular frequency in a high frequency stage) of 45.degree. between it and the video signal, a video signal of a waveform shown in FIG. 3A appears. On the other hand, if such signal is differentiated and inverted in polarity, a ghost detecting signal of a differentiated waveform shown in FIG. 3B is obtained. This differentiated waveform can approximately be regarded as an impulse response of the ghost signal.
The ghost detecting signal of differentiated waveform appearing from the differentiating circuit 17 is supplied through an amplifier 18 to demultiplexers 19 and 20 connected in series. The demultiplexers 19 and 20 are formed in the same way as in the delay circuits 8 and 11 wherein delay elements, each of which has a sampling period taken as a unit of delay, are connected in plural stages and m and n taps are led out from respective stages. The outputs from the respective taps are supplied to switching circuits 21.sub.1, 21.sub.2, . . . 21.sub.n and 22.sub.1, 22.sub.2 . . . 22.sub.m.
The vertical synchronizing signal from the synchronizing separating circuit 14 is supplied to a gate pulse generator 23 which then generates a gate pulse of a duration corresponding to the interval from the rising edge VE of the afore-said vertical synchronizing signal to the end terminal of the 1/2H interval. This gate pulse turns on the switching circuits 21.sub.1 to 22.sub.m.
The signals from the switching circuits 21.sub.1 to 22.sub.m are respectively supplied to analog accumulative adders 24.sub.1, 24.sub.2 . . . 24.sub.n and 25.sub.1, 25.sub.2 . . . 25.sub.m. The signals from the analog accumulative adders 24.sub.1 to 25.sub.m are respectively supplied to the weighting circuits 9.sub.1 to 9.sub.n and 12.sub.1 to 12.sub.m.
The outputs from the weighting circuits 9.sub.1 to 9.sub.n and 12.sub.1 to 12.sub.m are added together in an adding circuit 26 thereby forming a ghost cancelling signal. This ghost cancelling signal is then supplied to the composer 6.
As set forth above, the delay circuits 8, 11, the weighting circuits 9.sub.1 to 9.sub.n, 12.sub.1 to 12.sub.m and the adding circuit 26 constitute the transversal filter which serves to cancel out the ghost signal. In this case, the analog accumulative adders 24.sub.1 to 25.sub.m are provided such that after the distortion of the waveform in the period between the rising edge of the vertical synchronizing signal and the .+-.1/2H interval before and after it is detected and the weighting function is determined, if the ghost signal still remains not cancelled, the detection as afore-said is further carried out to reduce the ghost signal not yet cancelled.
The elimination of the delay-ghost signal can be changed to a feedforward mode, a feedback mode and vice versa by switching the mode switch 10.
FIG. 4 shows a case in which an input-adding type transversal filter is employed to eliminate a ghost signal. In the figure, like parts corresponding to those in FIG. 1 are marked with the same references and will not be described in detail.
In the figure, the video signal from the video signal detecting circuit 4 is supplied to the weighting circuits 9.sub.1 to 9.sub.n, and the signals from the weighting circuits 9.sub.1 to 9.sub.n are respectively supplied to input terminals of a delay circuit 8'. The delay circuit 8' is formed such that n delay elements, each of which has a sampling period as a unit of delay are connected and n input terminals are provided at portions between the respective stages.
The signals at the input and output sides of the composer 6 are supplied to terminals 10f' and 10b' of a mode switch 10'. The signal from the switch 10' is supplied to the weighting circuits 12.sub.1 to 12.sub.m and the signals from the weighting circuits 12.sub.1 to 12.sub.m are respectively supplied to input terminals of a delay circuit 11'. The delay circuit 11' is formed such that m delay elements, each of which has a sampling period as a unit of delay, are connected and m input terminals are provided at portions between the respective stages thereof.
The signals derived from the last stages of the delay circuits 8' and 11' are added together in an adding circuit 26' so as to form a ghost cancelling signal. This ghost cancelling signal is supplied to the composer 6.
This circuit can also eliminate the ghost signal in the same way as in the afore-said circuit having the output-adding type transversal filter.
Furthermore, in accordance with the afore-said circuit, without the differentiating circuit 17, a difference of outputs between the adjacent bits from the demultiplexers 19 and 20 is used to obtain a difference output and the weighting operation can be performed by the use of such difference output.
Moreover, if the demultiplexers 19, 20 and the delay circuits 8, 11 are used in common and upon determining the weighting amount the weighted signals are supplied to the delay circuits and then stored in memory elements, the weighting operation can be carried out by the stored signals thereafter.
In this way, the ghost signal can be eliminated in, for example, the video signal stage.
By the way, in such ghost signal cancelling apparatus, the formation of the standard waveform and the switching timing of the switching circuits 21.sub.1 to 22.sub.m are determined by, for example, the rising edge of the vertical synchronizing signal taken as a reference time. In that case, quite a high precision is required for the detection of the reference time and the experiment reveals that the precision must be within 35 nano seconds.
However, since the conventional synchronizing separating circuit includes therein a low-pass filter, a high frequency band range information is dropped and the rising edge or the like of the signal is blunted. As a result, if the reference time is detected from the vertical synchronizing signal thus separated, the reference time may be delayed.
Therefore, it has been proposed that a masking pulse of about 1/2H period long which includes the rising edge of, for example, the vertical synchronizing signal is formed and this masking pulse and the video signal are employed to directly detect the transit of the rising edge.
However, in the case of such previously proposed method, the influence of noise and so on causes an erroneous detection frequently. That is, when detecting the transit, a threshold level is provided to detect a time point at which the signal goes across this level. In general, this threshold level is selected as a half level between sync-tip and pedestal level. In that case, when a noise level is small as shown in FIG. 5A, quite a precise detection can be carried out. On the contrary, when a noise level is large as shown in FIG. 5B and this noise exceeds the half level, an error pulse occurs at each time so that the correct reference time cannot be detected.
The reason for this is that, since the influence of a synchronizing separating clamping circuit provided at the input side of the detecting circuit causes the sync-tip to pedestal portions to be enlarged enormously particularly when the electric field is weak and the noise level is also enlarged enormously in accordance therewith, a bad influence occurs easily.
In view of such aspects, the present invention is to provide a reference time detecting circuit for a video signal having a simple arrangement which can always detect a precise reference time.